Generalised epilepsy with febrile seizures plus (GEFS+; MIM 604236) was first described by Scheffer and Berkovic (1997) and is now recognised as a common epilepsy syndrome (Singh et al. 1999; Baulac et al. 1999; Moulard et al. 1999; Peiffer et al. 1999; Scheffer et al. 2000). Although GEFS+ is familial, it was initially difficult to recognise it as a distinct syndrome, because of clinical heterogeneity within each family. The common phenotypes are typical febrile seizures (FS) and febrile seizures plus (FS+); FS+ differs from FS in that the attacks with fever continue beyond age 6 years and/or include afebrile tonic-clonic seizures. Less common phenotypes include FS+ associated with absences, myoclonic or atonic seizures, and even more-severe syndromes such as myoclonic-astatic epilepsy. That such phenotypic diversity could be associated with the segregation of a mutation in a single gene was established with the identification of a mutation in the voltage gated sodium channel β1 subunit gene (SCN1B) (Wallace et al. 1998). This mutation (C121W) changes a conserved cysteine residue, disrupting a putative disulfide bridge, which results in in vitro loss of function of the β1 subunit. Without a functional β1 subunit the rate of inactivation of sodium channel α-subunits decreases, which may cause increased sodium influx, resulting in a more depolarised membrane potential and hyperexcitability. Modifier genes or the environment may interact with the SCN1B gene to account for clinical heterogeneity, but the rarity of SCN1B mutations (Wallace et al. 1998) strongly suggested additional genes of large effect underlie GEFS+ in other families (Singh et al. 1999).
GEFS+ in four families has been mapped to chromosome 2q (Baulac et al. 1999; Moulard et al. 1999; Peiffer et al. 1999; Lopes-Cendes et al. 2000). Recently, mutations in the neuronal voltage gated sodium channel alpha-1 (SCN1A) subunit were described in two GEFS+ families (Escayg et al. 2000). The mutations (T875M and R1648H) are located in highly conserved S4 transmembrane segments of the channel which are known to have a role in channel gating. It was suggested that these mutations may reduce the rate of inactivation of SCN1A and therefore have a similar effect as the β1-subunit mutation.
GEFS+ is clearly a common complex disorder, with a strong genetic basis, incomplete penetrance and genetic and phenotypic heterogeneity. Febrile seizures occur in 3% of the population, and thus this phenotype may occur sporadically in GEFS+ families, in addition to occurring as a result of the GEFS+ gene (Wallace et al 1998). Also, although some families segregate an autosomal dominant gene of major effect, in many cases clinical genetic evidence, such as bilineality, suggests that for some small families the disorder is multifactorial (Singh et al 1999). Despite this, large families continue to be ascertained and with critical phenotypic analysis, they provide opportunities to localise and ultimately identify the genes involved.